The present invention relates to transmission of information and in particular to methods and means for improving the quality of a signal which is detrimentally affected by degradation of a signal path.
During transmission of electromagnetic signals from a transmitter to a receiver via a signal channel, regardless of it being a fixed wire channel or a radio channel, the concept of signal deterioration must be considered. In very general terms, irrespective of whether or not the signal is representing analogue or digital information, there are detrimental effects due to noise, path loss and fading.
In the case of signals representing sequences of digital information there enters a further problem, that of time dispersion of the digital sequence. In particular, mobile communication systems are in most cases affected by the phenomenon of time dispersion. An understanding of time dispersion can easily be gained by grasping the concept of multipath reception. Multipath reception is experienced by a receiver, such as e.g. a mobile telephone, located at some distance from a transmitter, i.e. a base station antenna in the case of mobile telephone receiver, receiving an emitted signal via a multitude of geometrical paths through the atmosphere. The receiver may receive the signal directly from the transmitter as well as reflected from distant objects, such as hills or buildings. The main feature being that the signal paths are different in length.
The effect of the time dispersion on the digital signal sequence is that of so called inter symbol interference (ISI). Inter Symbol Interference means that consecutive symbols, i.e. bits in the sequence, interfere with each other making it difficult for the receiver to decide which symbol is actually transmitted by the transmitter. If a reflected part of the transmitted signal arrives exactly one bit time after a part of the signal which is received directly without being reflected, the receiver will detect a first symbol from the reflected part at the same time as it detects a second symbol from the direct part of the signal. If the symbols, i.e. bits, are different, e.g. the first symbol being zero and the second symbol being one the symbols interfere more or less destructively, and the receiver may be confused regarding the interpretation of the symbol which was actually received.
It is hence a problem of how to improve the quality of a signal, which has been subject to detrimental effects such as inter symbol interference.